My research focuses on the depositional environments and conditions that preserve vertebrate fossils. By documenting the stratigraphic distribution of fossils in the field and by evaluating taphonomic features of fossil bones in museum collections, I interpret the sedimentological and biological processes through which fossils accumulated. My current research focuses on the preservation of mammal fossils in Miocene sequences of the western United States, in particular, the Barstow Formation in California.
Taphonomy encompasses all processes that affect a fossil, from death of the organism to final burial. Features of fossil bones and aspects of the fossil locality contribute to our understanding of how animals lived and died. Bone breakage patterns, tooth marks, and abrasion are some of the features that allow us to interpret what processes contributed to the death of the animal or deposition of its remains. In the field, I document important aspects of fossil localities, including lithology, sedimentary structures and bed geometry, the number and type of specimens present, the spatial and vertical distribution of skeletal elements, and whether skeletal elements are articulated or associated. Describing these properties of the fossil setting are essential for interpreting its depositional setting.
A fragment of a camel mandible that was chewed by rodents.
Schematic representation of the factors affecting the preservation of fossils in the continental rock record.
The stratigraphic distribution of fossils is dependent on the distribution of sedimentary facies in a stratigraphic sequence. Sedimentary facies represent the specific depositional environments inhabited by plants and animals in life or the depositional environments that are able to preserve their remains.
In the marine record, the relationship between facies and organisms is straightforward: the ecological tolerances of many marine invertebrates are often related to water depth, and so organisms live along gradients characterized by changing substrates and increasing water depth. As sea level fluctuates over time, organisms shift their ranges along the gradient, and the shifting depositional environments are preserved as stacked sequences of facies. Typical sequences of stacked marine facies form as a function of fluctuations in relative sea level. Therefore, fossil occurrences can be predicted in a stratigraphic sequence when these facies-stacking patterns are recognized.
In the continental record, the relationships between organisms and habitat are less straightforward. Over regional scales, climate and tectonics are the dominant controls on geological processes, the distribution of ecosystems, and the geographic ranges of plants and animals within those ecosystems. The depositional environments that preserve the remains of plants and animals are much more restricted in extent, and are often highly localized. Recognizing the stratigraphic controls on fossil preservation in continental sequences is important for understanding the observed patterns of fossil distribution through time and space.
As paleontologists, we are interested in the ecosystems inhabited by extinct plants and animals, and we use many tools to reconstruct the environments represented at fossil localities. Soil organic matter and soil carbonate preserve the isotopic signature of vegetation and are commonly used to reconstruct vegetation composition (in particular C3 vs. C4 vegetation), vegetation structure (open or closed canopy), and relative precipitation amount. Other indicators include phytoliths (microscopic plant fossils) and stable isotopes from biomarkers (plant-wax residues) that are preserved in sediment. Phytoliths are amorphous silica that is precipitated in the cells of plants, and many plants produce morphotypes that can be identified to family or functional type. The phytolith assemblage from a sediment sample can then be used to reconstruct the structure and composition of the vegetation growing at the time of deposition. Using these tools and methods throughout a stratigraphic sequence allows us to study how paleoenvironments changed throughout the depositional history of a geological formation.
In the Barstow Formation, California, I used geochemical and micropaleontological proxies to reconstruct vegetation composition, vegetation structure, and moisture dynamics through the Middle Miocene Climatic Optimum (MMCO), an interval of global warming that occurred 17 to 14 Ma. I used carbon isotopes in normal-alkanes (long carbon chains in plant leaf wax) and soil organic matter from sediment to reconstruct vegetation composition and moisture through the formation. I used phytolith assemblages collected throughout the formation to reconstruct changes in vegetation composition and structure through time.
Example of a phytolith assemblage from the Barstow Formation, California. Scale bar is 25 μm.